The invention relates generally to turbomachines and more specifically to an arrangement and a method for mounting articulated turbine closure buckets in entry slots of rotor wheels of the turbomachines.
Rotors for turbomachines are often machined from large forgings. Rotor wheels cut from the forgings are often slotted to accept the roots of turbomachine buckets for mounting. As the demand for greater turbomachine output and more efficient turbomachine performance continues to increase, larger and more articulated turbomachine buckets are being placed into service. The loads exerted by the larger buckets have increasingly required that more sophisticated and expensive materials and alloys be used for the turbine buckets and rotor wheel.
Adjacent turbine buckets on a rotor wheel are typically connected together by some form of cover bands or shroud bands around the periphery to confine the working fluid within a well-defined path and to increase the rigidity of the buckets. The interlocking shrouds may often present interferences in assembling buckets on the rotor wheel.
Turbine buckets are often assembled onto the rotor wheel in a tangential direction. Tangential entry dovetail design requires an opening or notch around the periphery of the rotor wheel where the buckets are inserted radially and then slid tangentially into place. After the assembly of all regular buckets, the notch is filled with a closure bucket and two adjacent (auxiliary) buckets. The closure bucket is keyed to auxiliary buckets. Thus the two auxiliary buckets share the pull-load of closure bucket. Such closure bucket and auxiliary buckets and even the keys must often be fabricated from materials with higher strength properties to accommodate the load of the closure bucket. Higher strength-lightweight materials, such as titanium, that are used for the closure arrangements are more costly than the steel buckets conventionally used for the regular buckets. Also, when titanium is used for closure and auxiliary buckets, then balancing buckets also made of lightweight titanium are needed to balance the load over the rotor wheel.
FIG. 1 illustrates a prior art turbine closure arrangement 5 for a turbine wheel, including closure buckets 30 and auxiliary buckets 10 for tangential entry onto a rotor wheel. The auxiliary buckets 10 are mounted on a tangential male dovetail 20 of the rotor wheel 25 on each side of a closure slot 15. The closure bucket 30 is shown in position for mounting on the male dovetail 20 in the closure slot 15. Each auxiliary bucket 10 includes a vane 11 and a root 12 that includes a female dovetail complimentary (obscured) to the tangential male dovetail 20. The closure slot 15 includes notches 45 in the male dovetail 20 of the rotor wheel 25. The auxiliary buckets 10 and the remainder of buckets (not shown) are inserted over the notches 45 and then disposed tangentially around the periphery of the rotor wheel 25. The closure bucket 30 includes a vane 31 and a root 32 that includes tangs 33 arranged to ride over the notches 45 in the male dovetail 20 of the rotor wheel 25. The closure bucket 30 may also include axial holes 42 through the tangs 33 for locking to axial holes 43 of the male dovetails 20 of the rotor wheel 25 with retaining pins 46. The closure bucket 30 further includes semicircular slots 34 for accepting retaining keys 35. The closure bucket 30 may be lowered into the closure slot 15 and locked to the notches 45 with retaining pins 46. The closure bucket 30 may then be locked with auxiliary buckets 10. Such an arrangement may put undesirable stresses on the narrow notched portion 45 of the rotor wheel 25.
Accordingly, it would be desirable to provide a closure arrangement and method for use with the loading of tangential entry buckets onto rotor wheels for turbomachines that would avoid undesirable high stresses but yet could avoid the use of more-costly special materials.